1. Field of the Invention
The present invention generally relates to a surface acoustic wave filter and a filter device.
2. Description of the Related Art
In the mobile telephone devices and wireless LAN (Local Area Network) systems today, frequency bands, such as the 800 MHz band, the 1.9 GHz band, and the 2.4 GHz band, are used. However, as the amount of information to be transmitted and the demand for higher communication speed has increased, frequency bands used in communications are shifting to higher frequency bands such as the 5 GHz band. In this trend, there is an increasing demand for bandpass filters that operate in frequency bands of 5 GHz and higher.
Meanwhile, surface acoustic wave (SAW) filters have become indispensable as RF band filters and IF band filters for mobile communication devices such as portable telephones, because of their steep cut-off characteristics, smallness, lightness, and inexpensiveness.
A SAW filter is produced by processing a thin film, such as an aluminum film, to form comb-like electrodes on a piezoelectric substrate made of lithium tantalate crystal or the like. After a chip is cut out of the processed substrate, the chip is mounted on a ceramic package of several millimeters in size, and is then electrically connected to the ceramic package by a bonding wire or the like.
Ladder-type SAW filters having SAW resonators with comb-like electrodes connected in a ladder-like fashion are generally known as a means to obtain steep pass characteristics required in filters for portable telephone devices that exhibit low losses in a broad band. In terms of losses, ladder-type SAW filters are considered more suitable for the use in higher frequency bands than filters of other types. An example of such a ladder-type SAW filter is disclosed in “RF Band SAW Filters for Cellular Phones (Yoshio Satoh and Osamu Ikata, IEICE Transactions, Vol. 84, No. 11, November 2001, pp. 782-789)”.
In a ladder-type SAW filter, the electrostatic capacitance is adjusted by changing the aperture length and the number of electrode finger pairs in each of parallel-arm and series-arm SAW resonators connected in a ladder-like fashion, so that the input/output impedance can be matched with that of an external circuit. Japanese Unexamined Patent Publication No. 6-69750, for example, discloses a method of adjusting the electrostatic capacitances of SAW resonators to achieve excellent impedance matching. As the resonant frequency of the parallel-arm resonators and the antiresonant frequency of the series-arm resonators are substantially the same in the neighborhood of the center frequency under the condition of a constant K type filter, logical matching conditions can be expressed as:1/(ω02CopCos)=R2, ω0=2πf0  (1)
where f0 is the center frequency, Cop is the electrostatic capacitance of the parallel-arm resonators, Cos is the electrostatic capacitance of the series-arm resonators, and R is the nominal impedance. In bandpass filters for the RF units for wireless mobile communications, the nominal impedance R is normally 50 Ω.
Further, the optimum matching conditions that can be determined from the actual input/output impedance of a ladder-type SAW filter can be expressed as:
 −0.28Cos+3448/f0−746/f0≦Cop≦−0.28Cos+3448/f0+746/f0  (2)
Here, the center of the electrostatic capacitance Cop of the parallel-arm resonators can be expressed as:Cop=−0.28Cos+3448/f0  (3)
FIG. 1 shows the range of the electrostatic capacitances Cop and Cos that satisfy the matching conditions represented by the expression (2), with the center frequency f0 of the filter characteristics being 5.25 GHz. FIG. 2 shows the filter characteristics of a ladder-type SAW filter that was produced by setting the aperture length and the number of electrode finger pairs in each of the parallel-arm and series-arm SAW resonators in such a manner that the electrostatic capacitances Cop and Cos fall in the range indicated by the slanting lines in FIG. 1. In this example, aluminum (Al) with 1% copper (Cu) was used for the electrode film. FIG. 3 shows an equivalent circuit of this ladder-type SAW filter. In FIG. 3, each S represents a series-arm SAW resonator, and each P represents a parallel-arm SAW resonator.
As is apparent from FIG. 2, the center frequency in the filter characteristics of this ladder-type SAW filter was 5.2 GHz, the 4 dB bandwidth was 230 MHz, and the smallest insertion loss was 2 dB. Such a ladder-type SAW filter is disclosed in detail in “Development of Ladder-Type SAW Filter for 5 GHz Band (Tadashi Nakatani, Tokihiro Nishihara, Tsutomu Miyashita, and Yoshio Satoh, Proceedings of the 2001 IEICE ESS Conference, SA-3-9, September 2001, p. 289).
FIG. 4 shows the range of the electrostatic capacitances Cop and Cos that satisfy the matching conditions represented by the expression (2), with the center frequency f0 in the filter characteristics being 1.9 GHz. This ladder-type SAW filter has the same equivalent circuit as that of FIG. 3.
As is apparent from the above facts, the electrostatic capacitances of SAW resonators need to be made smaller, as the center frequency f0 of the filter becomes higher. When the ratio Cop/Cos of the electrostatic capacitance Cop to the electrostatic capacitance Cos, which is one of the essential design parameters, is set at a standard value of 0.5 in a 1.9-GHz-band filter, the electrostatic capacitance Cop is in the range of approximately 1 pF to 1.5 pF, while the electrostatic capacitance Cos is in the range of approximately 2 pF to 3 pF, as shown in FIG. 4. In a 5-GHz-band SAW filter, on the other hand, the electrostatic capacitance Cop is in the range of approximately 0.3 pF to 0.5 pF, while the electrostatic capacitance Cos is in the range of approximately 0.6 pF to 1 pF, as shown in FIG. 1.
With smaller capacitances, however, the stray capacitance (approximately 0.4 pF) that exists between a signal terminal and a ground terminal in a ceramic package to which the SAW filter chip is mounted, and the stray capacitance (approximately 0.1 pF) that exists between signal electrodes and ground electrodes in the SAW filter chip, have a greater influence on the impedance matching. As a result, the impedance matching becomes poorer and causes a problem of a greater insertion loss of the SAW filter.
Japanese Unexamined Patent Publication No. 11-205080 discloses a method of reducing stray capacitance. In accordance with this disclosure, the stray capacitance existing between a metallized region in a package and an electrode film formed on the metallized region is reduced by minimizing the metallized region. Japanese Unexamined Patent Publication No. 10-13183 discloses another method of reducing stray capacitance. In accordance with this disclosure, signal electrodes are located at a distance of 10 μm or longer from ground electrodes on a piezoelectric substrate, so that stray capacitance is reduced.
However, the quantity of stray capacitance that can be eliminated by any of the above methods is too small to solve the aforementioned problems. By the method disclosed in Japanese Unexamined Patent Publication No. 10-13183, the filter chip becomes larger with an increase in the distance between electrodes, and the wiring length increases accordingly, resulting in a greater loss due to a greater wiring resistance.